1. Field of the Invention
The present invention relates to an image forming apparatus such as a full-color printer, which adopts an electrophotographic system. In particular, the present invention relates to an arrangement of a conductive support member provided to support a cleaning member.
2. Related Background Art
FIG. 6 shows a four-color full-color image forming apparatus adopting a tandem system. In the image forming apparatus, four image forming units (image forming portions) are arranged in order from upstream to downstream along a rotation direction of a recording material bearing belt (recording material bearing member) 107 (direction of an arrow R7). That is, a first (yellow) image forming unit Pa, a second (magenta) image forming unit Pb, a third (cyan) image forming unit Pc, and a fourth (black) image forming unit Pd are arranged in the stated order. The first, second, third, and fourth image forming units Pa, Pb, Pc, and Pd are respectively provided with photosensitive drums 101a, 101b, 101c, and 101d that are each rotatable in the arrow direction (clockwise direction in FIG. 6). The photosensitive drums 101a, 101b, 101c, and 101d are respectively charged by charging rollers (chargers) 102a, 102b, 102c, and 102d. Next, exposure by exposing devices 103a, 103b, 103c, and 103d is performed. Then, through development by developing devices 104a, 104b, 104c, and 104d, toner images of yellow, magenta, cyan, and black are formed on respective surfaces of the photosensitive drums 101a, 101b, 101c, and 101d. The toner images of the respective colors are superimposingly transferred to a recording material P conveyed from a sheet feeding cassette 108 and borne on a surface of the recording material bearing belt 107 in succession by transferring chargers 105a, 105b, 105c, and 105d. Toner (transfer residual toner) that remains on the surfaces of the photosensitive drums 101a, 101b, 101c, and 101d after the toner image transfer, is removed by cleaning devices 106a, 106b, 106c, and 106d. On the other hand, the recording material P after the toner image transfer is separated from the recording material bearing belt 107 and is conveyed to a fixing device 109 at which the toner images are fixed onto the surface of the recording material P through heating and pressurizing.
In the image forming apparatus described above, the charging rollers 102a, 102b, 102c, and 102d, which are advantageous in terms of stability of charging, downsizing and simplification of the apparatus, and the like, are suitably used as chargers. As charging biases applied to the charging rollers 102a, 102b, 102c, and 102d, only direct current components are used in some cases and components including direct current components and alternating current components superimposed on each other are used in other cases. In addition, as to the developing devices 104a, 104b, 104c, and 104d, a developing device using a two-component developing method is described in Japanese Patent Application Laid-open No. 55-32060 A and Japanese Patent Application Laid-open No. 59-165082 A. With the two-component developing method, non-magnetic toner and magnetic carrier are borne on surfaces of developing sleeves. Then, through application of alternating electric fields to the developing sleeves as developing biases, electrostatic latent images on the photosensitive drums are developed with the toner.
In the image forming apparatus adopting the tandem system described above, the four image forming units Pa, Pb, Pc, and Pd are arranged along the rotation direction of the recording material bearing belt 107. So, there is a tendency in that the overall size of the apparatus increases. To downsize the apparatus, it is effective to arrange the respective image forming units Pa, Pb, Pc, and Pd in proximity to one another.
In this case, however, the charging roller of the image forming unit of the upstream side and the developing sleeve on the image forming unit on the downstream side are arranged in proximity to each other, which causes the following problem.
In the image forming apparatus described above, at the time of image formation, during charging of the photosensitive drum on the upstream side through application of a high-voltage charging bias to the charging roller on the upstream side, a high-voltage developing bias is applied to the developing sleeve of the developing device on the downstream side. Therefore, due to changing of a high voltage induced in the developing sleeve, noise occurs in the charging bias applied to the charging roller and unevenness of surface potentials of the photosensitive drum occurs, which leads to a problem in that density unevenness occurs in a final toner image. Note that such a problem occurs also in the case of an image forming apparatus that uses an intermediate transferring belt (intermediate transferring member).
It should be noted here that a counter measure is conceivable with which the charging unevenness is prevented by newly providing a shield between the charging roller on the upstream side and the developing sleeve on the downstream side, although in this case, the number of components increases and also downsizing is hindered.
It is therefore, an object of the present invention to provide an image forming apparatus including multiple image forming units, with which charging unevenness ascribable to disposal of upstream-side charging means and downstream-side developing means in proximity to each other is suppressed without adding any new components and downsizing is possible.